The composite N1 component to gaps in noise☆
Introduction
The temporal resolution of the auditory system is critical for speech perception and discrimination and for sound localization. Measuring auditory temporal resolution faces the problem of spectral changes that accompany changes in the time pattern of the sound. One way to overcome this limitation is to use interruptions in white noise, because the spectral content of white noise remains flat even when abrupt gaps are introduced. Thus, the threshold for detecting gaps in broadband noise has been used as a psychoacoustic measure of auditory temporal resolution. Gap detection threshold in comfortably loud broadband noise is typically 2–3 ms (Eddins and Green, 1995, Moore, 1997, Penner, 1977, Plomp, 1964, Zeng et al., 1999), increasing to 20 ms with noise levels near hearing threshold (Irwin et al., 1981, Zeng et al., 1999).
Gap detection is typically examined with bursts of white noise interrupted by gaps of silence. Such a complex stimulus includes gap onset (noise offset) followed by gap offset (noise onset), in addition to burst onset (e.g. Eggermont, 2000, Rupp et al., 2002). Recently, psychoacoustic and evoked potentials measures of auditory temporal processes were compared in normal-hearing individuals and in patients with auditory neuropathy using gaps in continuous noise (Michalewski et al., 2005). In normal subjects evoked potentials (N1/P2 components) were recorded in response to gaps as short as 5 ms in both active and passive conditions. Gap evoked potentials in the patients appeared only with prolonged gap durations (10–50 ms). There was a close association between gap detection thresholds measured psychoacoustically and electrophysiologically in both normals and in auditory neuropathy subjects. The study concluded that auditory evoked potentials to gaps in continuous noise can provide objective measures of auditory temporal processes.
The latter study (Michalewski et al., 2005) reported that when the cortical potentials to gaps were triggered from gap onset (offset of the noise), the N1 was broad compared to N1 to tone or noise onset. N1 consisted of two separate components in most subjects when gap durations were longer than 20 ms: an early component peaking at 90 ms, similar in latency to a stimulus onset N1 and a later component peaking at approximately 150 ms. At gap durations of 20 ms or shorter, the N1 consisted of a single component approximately centered between the earlier (90 ms) and the later (150 ms) components. The later of the two peaks was usually the larger and frontally prominent.
Two possible explanations to account for the double N1 peak to gap onset (noise offset) were proposed (Michalewski et al., 2005): One explanation attributed the later of the two peaks to a separate perceptual distinction made by the subjects to longer gap durations (Phillips, 1999). Thus, given its scalp distribution (frontal) and timing (approximately 150 ms) the second peak may be attributed to a mismatch negativity to stimulus change (Naatanen, 1992) or a negative component related to auditory change, an ‘acoustic change process’ (Jones and Perez, 2002). The other explanation included the possibility that the double peaked N1 may involve a combination of offset/onset responses, or the interaction of these responses in the averages. Neither of these alternative explanations could be ruled out.
The purpose of this study was to indicate whether the early ERP components to onset and to offset of gaps in continuous white noise are simple composites of the well known onset and offset responses to transients or whether they also reflect a more advanced process such as change detection. Therefore, in this study potentials to gap onset and offset were separated and compared with the better studied potentials to short transient stimuli (clicks). Potentials were recorded while subjects attended or ignored the stimuli, and compared to assess the effect of attention on these components.
Section snippets
Subjects
Thirteen, right handed, normal hearing subjects, 18–25 years old participated in the study. Subjects were paid for their participation and all procedures were approved by the institutional review board for experiments involving human subjects (Helsinki Committee).
Stimuli
Binaural stimuli were used throughout this study to avoid confounding the scalp distribution of evoked potentials by contralateral or ipsilateral stimulation. Thus, any lateralization of brain activity would be attributed to
Behavioral gap detection
Reaction times and performance accuracy of gap detection for the gap durations of this study are presented in Fig. 2. In general, accuracy levels for gaps of 10 ms and longer approached 90% on average, dropping to about 60% with gaps of 5 ms duration and further dropping to below chance with the shorter gaps. This effect of gap duration on performance accuracy was significant [F(5,55)=117.77, P<0.0001]. Reaction times were around 500 ms for gaps 10 ms and longer, approaching 600 ms for gaps of 5 ms
Discussion
In this study potentials to short gaps in noise and to onset and offset of long gaps in noise were compared with the better studied potentials to short transient stimuli (clicks), while subjects attended or ignored the stimuli. Many of the findings of this study corroborate the findings of an earlier study (Michalewski et al., 2005). Thus, no effects of attentional state on the N1–P2 complex were observed, and amplitudes and latencies of the constituents of this complex were the same, whether
Acknowledgements
Henry Michalewski and Arnold Starr introduced us to the intriguing complexity of evoked potentials to gaps in continuous noise. Discussions with Ilan laufer contributed to our appreciation of the processes associated with N1a and N1b. This study was partially supported by the Rappaport Family Institute for Research in the Medical Sciences.
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2018, Trends in NeurosciencesCitation Excerpt :However, in an animal model of gap-detection deficits, auditory thalamic abnormalities in gap-in-noise sensitivity have been linked to specific deficits in offset responses, whereas onset responses appear normal [25] (Box 2). Moreover, in humans, cortical event-related potentials evoked by brief gaps in noise include a distinctive N1 component that is specifically related to the cessation of an ongoing sound [40]. Thus, the neural basis for gap detection remains in question.
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Some of these results were presented at the Eighth International Evoked Potentials Symposium, October 2004, in Fukuoka, Japan.